Conventional infrared receiver can be typically classified into two types, one of them directly outputs the modulated or carrier signal from the infrared receiver as the output signal and thus for application it should be incorporated with an additional carrier-filtering circuitry such as an intermediate frequency (IF) filter combined with a detector to filter out the carrier component from the output signal of the infrared receiver, as shown in FIG. 1. In FIG. 1, the modulated or carrier signal from the infrared photodiode receiver front-end 10 is amplified by an amplifier 12 and then filtered by an IF filter 14, resulting in a larger distortion, a detector 16 and a waveform shaper 18 extract the rectangular wave and amplify it, and finally the data is outputted. The other type of infrared receiver is an infrared receiver module 20 as shown in FIG. 2, which comprises an amplifier and a carrier filter within the module 20 and is able to directly output a digital data signal.
Among the above two types of infrared receivers, the second one is more convenient for utilization since no additional carrier-filtering circuitry is necessary, however, it comes with higher cost, causing users to choose the first type and combine a carrier-filtering circuit by themselves.
On the other hand, for conventional receiver circuit available for both infrared and ultrasonic transmission, in addition to a built-in amplifier to amplify the carrier signal thereof, there is also included an analog filter to filter out the carrier component from the carrier signal, and the analog filter employed is constructed with resistors, capacitors, diodes and inductors, thereby the analog filter is; not suitable for integrated circuit.
However, digital filtering has been developed to demodulate signals, for instance, by Taiwan paten application no. 83213974 entitled “Improved Infrared Receiver” issued to Cheng. Cheng proposed an apparatus to do signal filtering by a digital filter to avoid the distortion and error resulting from the filtering process, thereby increasing the signal reliability and accuracy.
Nevertheless, a cheap receiver available for infrared and ultrasonic transmissions with digital filtering is not disclosed. The present invention is therefore directed to an improvement of an integrated circuit infrared and ultrasonic receiver employed with digital filtering technique.